1. Field of the Invention
This invention relates to stable organic solutions of polyimide-forming monomers having improved shelf-life and more specifically to the process of manufacturing, shipping, handling, storage and the fabrication layup of all types of PMR (polymerization of monomeric reactants) polyimide-forming monomeric solutions and the PMR polyimide prepregs derived therefrom without adversely affecting subsequent processability at cure temperature of the PMR resins and PMR composites.
2. Description of Related Prior Art
Polymerization of Monomer Reactants (PMR) to obtain polyimides is an important class of ultra high performance composite resins. Polyimide graphite fiber reinforced composites are increasingly used in various aircraft engine components, which operate at temperatures ranging up to 371.degree. C. for thousands of hours. For example, PMR-15 is one of the best known and most widely used PMR polyimide. PMR-15 attributes include relatively easy processing, substantially lower costs, and excellent property retention at elevated temperatures, compared to other commercially available high temperature resin materials.
The preparation of polyimides from mixtures of monomeric diamines and esters of polycarboxylic acids is disclosed, for example, in U.S. Pat. No. 3,745,149. Patentee disclosed that polyimides can be processed from a mixture of monomeric reactants using lower (primary) alcohols to esterify an anhydride endcap and an aromatic dianhydride. These monomeric reactants when combined with an aromatic diamine in the molar ratio of N diester-diacid/N+1 diamine/2 ester-acid endcap, form a monomeric mixture which at high temperature polymerizes to a polyimide. This procedure was the evolution of the terminology PMR (polymerization of monomeric reactants). The initial concept of using lower (primary) alcohols to prepare methyl or ethyl ester-acids remains in use as originally disclosed in the art over the last twenty-five years.
Subsequently, however, few PMR patents have issued that improve over the prior art and generally these patents required a new "wrinkle" such as the use of monofunctional additives or new formulations using new dianhydrides, diamines or endcaps. These prior art patents are still using PMR technology based on methyl or ethyl ester-acids or diesters-diacids formed from lower (primary) alcohols. There is no prior art specifically covering the PMR Extended Shelf Life Technology obtained by the use of higher (secondary) ester-acids and higher (secondary) diester-diacids, as taught by this invention. Until now, all the prior PMR art remains evolved around the use of lower (primary) ester-acids in contrast to the benefits obtained by the use of the higher (secondary) ester-acids in PMR Extended Shelf Life Technology.
The major disadvantages of the state-of-the-art PMR technology, as practiced commercially today, are the limited shelf life of the monomeric solutions at ambient (room) temperatures, the short working outlife time, and an extremely high sensitivity toward premature aging at temperatures even slightly above room temperature. The disadvantages cause premature polymerization during all phases of PMR usage such as in synthesis, manufacturing, shipping, handling, storage, and fabrication layup/processing. The PMR technology employed today still uses the lower (primary) methyl and ethyl diester-diacid and ester-acid of the dianhydride and nadic anhydride endcap respectively. This methyl and ethyl ester technology is PMR's inherent weakness in that imidization proceeds rapidly at about room temperature, thereby quickly aging all PMR types of polyimides such that aged solutions and prepregs expire with limited shelf life within one to three weeks at room temperature thereby being unprocessable with autoclave fabrication techniques. The engineering solution, as opposed to the chemical solution, to premature aging of PMR solutions and prepregs has been through rigorous handling requirements via strict manufacturing temperature control, overnight air shipment in dry ice, freezer storage of received PMR materials and stringent quality control governing allowed outlife usage time and freezer storage time; all of which significantly adds to the final cost of PMR composites.
Another disadvantage of the state-of-the-art PMR technology is the use of toxic lower (primary) methanol and ethanol for esterification and as the solvent for PMR monomer solution preparation and PMR prepreg manufacturing. These primary alcohols create highly toxic volatiles for both the manufacturer and user to control. In comparison, the PMR Extended Shelf Life Technology of this invention only uses the higher (secondary) and much less toxic isopropyl alcohol for the esterification and solvent in the PMR monomer solution preparation and PMR prepreg manufacturing. The evidence of reduced toxicity and increased safety are a lower odor threshold, well before you reach a much higher allowed threshold limit value, an increased autoignition temperature and narrower flammability limits along with less serious medical problems of overexposure in the use of isopropanol, for example, in comparison to the use of methanol or ethanol.
The PMR extended shelf life technology of this invention is based on a chemical solution to significantly retard aging of PMR solutions and PMR prepregs, rather than on an engineering solution of rigid temperature control as evolved and still practiced in present PMR technology. The chemical solution to these problems is the use of the higher (secondary) C.sub.3 to C.sub.5 alcohols, e.g. isopropyl for esterification of the anhydride endcaps and dianhydride monomers.